-LRB- CNN -RRB- ISIS militants kidnapped 20 foreigners working at a Libyan hospital , then released them -- under the condition , if they want to live , that they stay put so they can treat members of the Islamist extremist group , a hospital official said .

About 30 gunmen tied to the group calling itself the Islamic State stormed Ibn Sina Hospital in Sirte on Monday while a bus was waiting to take the workers to Tripoli , Libya 's capital .

The medical workers were later released and sent back to their homes near the medical facility , a hospital official said Tuesday . But they ca n't go far , with ISIS militants ordering them not to leave Sirte , according to the official .

One of those kidnapped , a doctor from Uzbekistan , was told he is safe as long as he did n't leave , and he treated any militants who were wounded , the hospital official said .

`` They told him that , for your life , you -LRB- stay -RRB- and work in the city , '' the official added .

Like the doctor , the other medical workers are n't Libyan . Most are from the Philippines , with others from Ukraine , India and Serbia .

The kidnappings came days after people of Filipino , Austrian , Czech , Ghanaian and Bangladeshi descent were taken from Libya 's Al-Ghani oil field , an operation that Libya 's internationally recognized government blamed on `` ISIS militias . ''

The abductions are more evidence of the turmoil Libya has experienced since 2011 , the start of an uprising against longtime dictator Moammar Gadhafi .

Sirte was Gadhafi 's hometown , where he was killed , and where his loyalists held out the longest . In the years since , Sirte has become a home to ISIS .

The coastal city , halfway between Tripoli and Benghazi , is where terrorists kidnapped 21 Coptic Christians from Egypt in separate incidents in December and January .

That mass abduction was followed by mass slaughter -- one that , in ISIS ' distinctive , depraved style , was videotaped and disseminated as propaganda , showing jihadists standing behind their orange-clad , handcuffed victims and then beheading them .

`` The sea you have hidden Sheikh Osama bin Laden 's body in , '' a masked English-speaking jihadi said on the video , `` we swear to Allah , we will mix it with your blood . ''

Like the Egyptians , the medical workers kidnapped Monday are foreigners , having come to Sirte to work .

They also have skills that could help ISIS .

The Ibn Sina Hospital official believes the extremist group went after the workers -- who 'd been trying to flee Sirte 's precarious security situation -- because they make up the only medical team there and might be needed to treat ISIS militants .

The large-scale kidnapping illustrates how ISIS has become a disruptive force in Africa .

The group 's main foothold is in Iraq and Syria , where it has ruthlessly conquered territory and threatened to take more despite efforts by local governments and a U.S.-led air campaign .

ISIS branched out into Libya last year , with CNN reporting in November that fighters loyal to the group had complete control of Derna , a city of about 100,000 near the Egyptian border .

Militants who 've pledged allegiance to ISIS also have made their mark in points westward , forming chapters in cities including Sirte , Benghazi and Tripoli , according to Noman Benotman , a former Libyan jihadist now involved in counterterrorism as head of the Quilliam Foundation .

Besides its actions in Derna and Sirte , ISIS ' Libyan branch claimed responsibility for a January attack on a luxury hotel in Tripoli that killed 10 people , one of them American David Berry .

But ISIS is n't just active in Libya . Earlier this month , Boko Haram -- another Islamist extremist group with its own reputation for mass abductions and craven brutality -- pledged allegiance to ISIS leader Abu Bakr al-Baghdadi .

This bond gives ISIS a gateway to West Africa and specifically to Nigeria , Africa 's most populous nation and Boko Haram 's home .

@highlight

ISIS tells doctor to stay , treat militants , hospital official says

@highlight

About 30 gunmen stormed the Sirte hospital , took 20 foreigners captive

@highlight

ISIS has been active in Sirte , where it also abducted Christians from Egypt